v3.9.2 — docs reorg: per-service pages under docs/services/

Patch release. Pure documentation restructure — no production code
changes.

The two monolithic doc files were growing unmanageable:
  docs/services-reference.md  ~960 lines / 18 service sections
  docs/usage.md               ~780 lines / 15 task-keyed sections

Both have been split. Every service now has ONE file under
docs/services/<name>.md containing both the deep reference (endpoints,
models, config, internals) and the curl recipes / usage examples.

18 new per-service files + 2 new index pages (docs/README.md +
docs/services/README.md). Old files now redirect to the new structure.
README + docs/providers.md links migrated to the new paths.

See CHANGELOG.md for the full mapping of old section → new file.